In the development of electronic systems that include integrated circuits and other components, various types of prototyping boards have been used. These boards include perforated bread boards without conductors, selectively perforated multilayer printed boards, and customized lay-out boards. To accommodate various electronic component packages, a prototype board contains a pattern of perforations that accept component pins for mechanical and electrical interconnection. It is desirable to have an array of such perforations so that the prototyping board can accept a wide variety of electronic component lead spacing.
In the prototyping of high-speed circuits, it is desirable to prototype the circuit on microstrip or stripline. These transmission line media are very popular for short distance, low power applications, including RF microwave circuits and high speed digital circuits. Stripline consists of a printed conductor between two ground planes, typically formed from copper-clad fiberglass sheets. Electrically, stripline has properties similar to coaxial cable transmission lines. Microstrip, a popular transmission line due to ease of fabrication and circuit assembly, uses dielectric substrates with a metal deposited on or etched way to form the circuit line conductors. Microstrip has a single ground plane with a dielectric layer sandwiched between the circuit conductors and the ground plane. Because the circuit conductors are exposed, microstrip is easily used for prototyping high-speed circuits. Although stripline provides inherently better crosstalk performance because the field within the stripline conductor is totally contained between the two ground planes, it has not heretofore been possible to prototype circuits using stripline. This is obvious from the inherent stripline design, wherein it is impossible to access the inner circuit conductors once the stripline board has been fabricated.
In the typical prototyping operation a stripline board is laid out, given due consideration to signal path lengths and interaction between neighboring components. The electronic components are mounted on the prototype stripline board and the performance of the system is observed. If the performance is not adequate, another stripline prototype board is constructed, using the information gained from the unsuccessful lay-out. The components are then attached to the second stripline board and the system is again tested. It is obvious that this iterative approach to devising a working stripline layout suitable for board and circuit production is an expensive and time-consuming process. Further, once a suitable prototype board has been produced, unless the production board is an exact duplicate thereof, the final production circuit may not perform identically to the prototype layout.
In lieu of having another prototyping stripline board constructed, it is also well-known to change the stripline board by simply cutting the ground planes, dielectric layers, and circuit conductors to change the circuit interconnections and component locations. It is also possible to connect components by simply running semi-rigid coaxial cable outside the stripline ground planes. While these techniques allow the stripline board to be changed easily, such a modified stripline board does not have the same crosstalk and signal-to-noise ratio performance as does a stripline board having complete ground planes with all circuit conductors contained therebetween.
It would thus be desirable to provide a three-layer (or stripline-like) prototyping board on which electronic components could be easily mounted and relocated as desired. Such a board would reduce the costly and time-consuming operation of fabricating a new prototyping three-layer board for every circuit change. Such a prototyping board should also desirably facilitate the transfer of the prototyping layout to a production layout.
In U.S. Pat. No. 4,695,810 issued Sept. 22, 1987 entitled, "Waffleline-Configured Microwave Transmission Link", and assigned to the assignee of the present application, there is described a miniaturized transmission link for microwave and high-speed circuit applications. The invention provides a scheme for intercoupling high-frequency miniaturized integrated circuit components that provides an extremely compact universal interconnection architecture and provides a constant transmission line impedance along the link, irrespective of the placement of the components within the architectural structure. The invention comprises a thin conductive plate in one surface of which rectilinear grooves or channels are formed. The grooves are formed as a matrix grid work of mutually orthogonal channels, creating a "waffle-iron" like pattern in one surface of the conductive plates. The spacing between channels corresponds to the width of a channel which, in turn, may be sized to substantially match the outer diameter of insulation jacketed wire that is placed in the channels. The depth of a channel or groove is slightly larger than the outer diameter of the wire to accommodate wire crossovers at intersections of the channels. The top surface of the "waffle-plate" is provided with a conductive foil or plate to complete the shielding for the wires.
The conductive (e.g., aluminum) plate is initially etched or machined (e.g., through the application of parallel-spaced apart saw cuts) so that the entire surface of one side of the plate has a waffle-iron configuration. Then, where miniature circuit components are to be provided, selected portions of the waffle-iron structure are further removed to leave pockets for receiving the components. The pockets may be sized to accommodate high-density leadless chip carriers whose input/output port connections are substantially aligned with the channels of the waffle-iron structure, thereby greatly facilitating their interconnection through the channels of the waffle-iron structure. This invention also provides a feed-through connection so that components can be mounted on the bottom surface of the waffle-iron sheet area. For example, a connector can be mounted on the bottom surface of the sheet and a signal conductor having an inner conductor surrounded by an outer dielectric jacket can be routed through a hole in the waffle-iron sheet. A circular aperture is then provided in the waffle-line plate in a direction orthogonal to the channels. The insulated signal conductor is routed through the aperture for connection to a signal line routed within the channels or grooves.
While the waffleline invention provides an improved high-speed transmission line connection technique, it is not suitable for prototyping of experimental high-speed circuits. Components must be mounted by either removing a portion of the waffle-iron structure to create a hole suitable for receiving the component, or apertures must be drilled in the waffleline structure at locations where components are to be mounted. Thus the waffleine invention does not provide the flexibility necessary for quickly and efficiently breadboarding and later changing an experimental circuit.